PLAINS ALL AMERICAN PIPELINE LP

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549​

SCHEDULE 14A

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the Securities Exchange Act of 1934 (Amendment No. )

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Definitive Proxy Statement

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Soliciting Material under §240.14a-12

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PLAINS ALL AMERICAN PIPELINE, L.P.

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April 10, 2026​

Dear Fellow Plains Investors,

We sincerely appreciate your continued support and investment in Plains. Your trust and partnership play a vital role in our success, and we are pleased to invite you to join us for our 2026 Annual Meeting.

2025 was a year marked by a complex macro backdrop with increased volatility, global uncertainty and OPEC+ supply increases all resulting in softening markets and a lower than expected oil price environment. Plains executed well during 2025 despite this challenging environment, managing what was in our control and delivering on our capital allocation framework and efficient growth strategy. We proactively executed several major transactions and self-help initiatives to position the partnership as the premier North American crude oil midstream company and better prepare Plains for the increased global volatility and uncertainty that has characterized markets for the first four months of 2026.

Notable 2025 accomplishments included:

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Reached an agreement to sell our Canadian NGL business to Keyera at an attractive valuation of ~$3.75 billion, with closing targeted for May 2026.

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Proactively redeployed the anticipated NGL sale proceeds to acquire the EPIC crude system, renamed Cactus III, for ~$2.9 billion in a transaction that was immediately accretive to distributable cash flow (DCF) and provided Plains with a higher quality and more durable cash flow stream.

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Progressed our efficient growth strategy and completed five accretive bolt-on acquisitions for an aggregate purchase price of approximately $800 million net to Plains.

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Accelerated streamlining and optimization initiatives that are expected to result in ~$100 million of cost savings by the end of 2027, with approximately half to be realized in 2026.

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Increased our annualized distribution by $0.15/unit (10%) in February 2026 and delivered strong total unitholder and shareholder returns in 2025 of 14% and 13% for PAA and PAGP, respectively.

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These accomplishments position Plains well for continued global volatility and uncertainty and frame 2026 as a year of execution for Plains. During 2026, we will focus on closing the sale of our Canadian NGL business, capturing ~$50 million of synergies on the Cactus III pipeline, and realizing ~$100 million of cost savings through 2027 (with ~$50 million recognized in 2026), while successfully navigating through the market uncertainty, capturing opportunities and limiting downsides. Execution of 2026 initiatives is expected to result in ~13% year-over-year growth in the crude segment, despite a relatively flat Permian production outlook for 2026. Moving into 2027, we expect growth to resume in the Permian based on improving fundamentals, and we believe Plains is well-positioned to capture value with a premier footprint and operating leverage.

Recent geopolitical conflicts in the Middle East highlight the critical role the U.S. energy industry plays as an essential provider of global energy reliability, affordability and security. With our network of strategically located North American midstream assets and our focus on the long-term optimization of our business, we believe we are well positioned to play a critical role in the delivery of energy across North America and globally. As we do so, we will remain focused on our key objectives of being a safe, reliable and responsible operator, generating strong cash flow, exercising capital discipline, increasing returns of capital to our investors, and maintaining financial flexibility. We believe the execution of our plan will deliver strong performance and enhanced value for our investors over the long term.

We appreciate your continued investment and support and we look forward to your participation in our 2026 Annual Meeting in May.

Sincerely,

Willie Chiang

Chairman of the Board, CEO and President

333 Clay Street, Suite 1600

Houston, Texas 77002

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NOTICE OF ANNUAL MEETING OF UNITHOLDERS

To Be Held On May 20, 2026

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To holders of common units and Series A Convertible Preferred units of Plains All American Pipeline, L.P.:

The 2026 Annual Meeting (the “PAA Annual Meeting”) of the common unitholders and Series A Convertible Preferred unitholders (collectively, our “Unitholders”) of Plains All American Pipeline, L.P. (“PAA”) will be held on May 20, 2026, at 10:00 a.m. Central Time, in the Texas Room located on the 19 th Floor of Three Allen Center, 333 Clay Street, Houston, Texas 77002. At the PAA Annual Meeting, our Unitholders (excluding Plains AAP, L.P.) will consider and vote on the following matters:

The election of four Class I directors to serve on the board of directors (the “Board”) of PAA GP Holdings LLC until the 2029 annual meeting;

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The ratification of the appointment of PricewaterhouseCoopers LLP as our independent registered public accounting firm for the fiscal year ending December 31, 2026;

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The approval, on a non-binding advisory basis, of our 2025 named executive officer compensation; and

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Any proposal to transact such other business as may properly come before the PAA Annual Meeting and any adjournment or postponement thereof.

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The Board unanimously recommends that our Unitholders vote “FOR” the election of each director listed in proposal 1 and “FOR” proposals 2 and 3. Additional information regarding these proposals is included in the attached proxy statement. Votes received on proposals 1, 2 and 3 will be “passed through” as voting instructions to PAA, the owner of all of the Class C shares of Plains GP Holdings, L.P. (“PAGP”), who will in turn vote such shares at PAGP’s annual meeting on behalf of our Unitholders pursuant to such instructions. See “Information About the PAGP and PAA Annual Meetings” below for more information.

Unitholders as of the close of business on March 23, 2026 (the “Record Date”) are entitled to receive notice of and to vote at the PAA Annual Meeting and any postponements or adjournments thereof. A list of Unitholders entitled to vote is on file at our principal offices, 333 Clay Street, Suite 1600, Houston, TX 77002.

Your vote is very important. Whether or not you plan to attend the PAA Annual Meeting, please cast your vote by following the Internet or telephone voting instructions on the proxy card. You may also vote by completing, signing and dating the accompanying proxy card and returning it promptly in the postage-prepaid envelope provided. See “Questions and Answers About the PAA Annual Meeting — How do I vote?” in the attached proxy statement for more details. Returning the proxy card or voting on the Internet or by telephone does not deprive you of your right to attend the PAA Annual Meeting and to vote your units for the matters to be acted upon at the PAA Annual Meeting.

By Order of the Board of Directors of

PAA GP Holdings LLC, general partner of

Plains GP Holdings, L.P., sole member of

Plains All American GP LLC, general partner of

Plains AAP, L.P., sole member of

PAA GP LLC, general partner of

Richard McGee

Executive Vice President, General Counsel and Secretary

Houston, Texas

April 10, 2026